1. Technical Field
The present invention relates to a hydraulically driven, dynamic tensile testing device for carrying out tests such as measuring the adhesive properties of a coating on a substrate or tensile testing materials that possess little elasticity.
During tensile testing, it is of vital importance that the tensile forces are evenly distributed over the cross-section of the test sample. A slight uneveness can result in that only a part of the cross-section of the sample is subjected to the measured forces. This could result in excessively low measured values for the tensile strength of the test specimen and measurements that are barely reproducible. Elastic test samples can compensate for uneven tension to a certain extent, but in the case of thin test samples, such as when measuring the adhesion of a coating to a substrate and when tensile testing inelastic materials, the materials will not be able to even out any uneven loads.
Tensile measuring of materials possessing little elasticity is therefore performed with extremely expensive specialized equipment operated by highly qualified personnel who have been especially trained.
The adhesion of a coating, e.g., paint, to an underlying surface is, for many uses, of vital importance. An inadequate adhesion to the underlying surface can result in the loosening and peeling off of the coating, and thus the loss of the protection which the coating is supposed to provide the underlying surface. Therefore, a minimum requirement for such adhesion is often made, a requirement that must be capable of being checked with reproducible and reliable measurements. The problems are often the same for a coating as for materials of little elasticity. An uneven load cannot be compensated for by the elasticity of the coating as the coating is relatively thin.
2. Prior Art
A number of devices for measuring the adhesion of a coating to a substrate are known. All the devices are based on a test piston fixed by glue to the substrate being pulled vertically upwards therefrom. The force that is needed to cause the coating to come away from the substrate gives a measurement of the adhesion of the coating to the substrate. In order to provide reliable and reproducible measurements, it is important that the tractive force is vertical to the surface and is evenly distributed over the test surface. If the tractive force is not vertical to the substrate, this may result in the coating being ripped up from one side, thereby giving inaccurate and non-reproducible measurements of the adhesion.
GB 1 179 149 makes known a device which is equipped with a torque arm and is therefore supplied with torsional force which is difficult to avoid. This device is also equipped with legs that can be adjusted by means of nuts in order to arrive at vertical tractive force. However, it can be difficult or even impossible to adjust the legs with the accuracy required in order to obtain an even distribution of the forces on the test surface, namely a deviation of less than a few .mu.m.
SE 379 243 makes known a device that is revolvably mounted on two legs. This device provides a relatively reliable measurement result, but it is large, heavy and dependent upon the use of a gas cylinder, and this makes the device rather unsuitable for use in the field.